CVE-2026-44798: Nautobot: GitRepository.current_head field should not be writable through REST API

A user with access to add/change a GitRepository record could use the REST API to directly set the current_head field on the record, which was not intended to be user-editable. Doing so could cause Nautobot’s local clone(s) of the relevant repository to checkout a commit other than the latest commit on the specified branch (resulting in misleading state), or potentially to be unable to make use of the repository at all (until manually remediated) due to the current_head pointing to a nonexistent commit hash or malformed value.